The invention relatees to a method of enciphering information-containing digital data signals with the aid of enciphering keys and of deciphering a message thus enciphered, utilizing the same key.
Such a method is disclosed in, for example, European Patent Specification 0011615. The key usually consists of a long sequence of binary numbers (ones and zeroes). Cryptographic text is obtained by adding the key (mostly exclusive-OR per bit) to the clear text to be enciphered. This cryptographic text is transferred or transmitted to a receiver where the original, clear text is obtained by subtracting the same key as utilized in the transmitter from the received cryptographic text. For uses where very high demands are imposed on the quality of the enciphering this method has many shortcomings.